A hydrogenated aromatic vinyl compound polymer that is obtained by hydrogenating the aromatic ring of an aromatic vinyl compound polymer, and a hydrogenated block copolymer that is obtained by hydrogenating the double bonds (that are derived from an aromatic ring and a diene) of a block copolymer that includes a polymer block (a) that includes a repeating unit derived from an aromatic vinyl compound, and forms a hard segment, and a polymer block (b) that includes a repeating unit derived from a linear conjugated diene compound and forms a soft segment, exhibit significantly improved transparency, low birefringence, and heat resistance as compared with a hydrogenated block copolymer that is obtained by selectively hydrogenating only a double bond derived from a diene, and are useful as an optical film (e.g., polarizing film and retardation film) (see Patent Literature 1 to 5).
Patent Literature 1 and 2 disclose using a stretched film that is obtained by stretching a film that includes a hydrogenated block copolymer obtained by hydrogenating double bonds derived from an aromatic ring and a diene, and is provided with the desired retardation, as a liquid crystal display retardation film.
However, the stretched films disclosed in Patent Literature 1 and 2 decrease in retardation when subjected to a heat resistance test at 80° C. (that is commonly used for a liquid crystal display optical film) for about 48 hours (i.e., do not exhibit sufficiently high heat resistance), and are considerably limited as to the service temperature.
Patent Literature 3 discloses a stretched film that is obtained by stretching a film that includes a hydrogenated triblock copolymer and provided with retardation, the hydrogenated triblock copolymer having a total content of a repeating unit derived from styrene (i.e., aromatic vinyl compound) of 87 wt %, wherein the hard segment is a polymer block formed of styrene, and the soft segment is a copolymer block that includes styrene and isoprene (i.e., linear conjugated diene compound).
However, the hydrogenated block copolymer disclosed in Patent Literature 3 has a glass transition temperature (hereinafter may be referred to as “glass transition temperature Tg”) measured by DSC analysis of 127° C., and does not exhibit sufficiently high heat resistance.
Patent Literature 4 discloses a retardation film that is obtained by stretching a hydrogenated pentablock copolymer that has a total content of a repeating unit derived from styrene of 60 wt %, and includes a polymer block formed of styrene, and a polymer block formed of butadiene. Patent Literature 4 does not disclose the glass transition temperature Tg of the hydrogenated pentablock copolymer. A hydrogenated pentablock copolymer that was synthesized using the method disclosed in Patent Literature 4 had a glass transition temperature Tg of 120° C. or less, and does not exhibit sufficient heat resistance.
Patent Literature 5 discloses a hydrogenated block copolymer wherein the soft segment is a copolymer block that includes a linear conjugated diene compound and an aromatic vinyl compound, wherein the content of the aromatic vinyl compound is lower than that of the linear conjugated diene compound. Patent Literature 5 states that the hydrogenated block copolymer exhibits improved thermal deformation resistance, mechanical properties, transparency, and the like. The hydrogenated block copolymer disclosed in Patent Literature 5 has a total styrene content of 85 wt %, includes a polymer block formed of styrene as a hard segment, includes a copolymer block formed of styrene and isoprene as a soft segment, has a glass transition temperature Tg1 measured by dynamic mechanical analysis (DMA) of −33° C., and has a glass transition temperature Tg2 measured by DMA of 132° C. The glass transition temperature Tg1 of the soft segment included in the hydrogenated block copolymer disclosed in Patent Literature 5 is higher than that (−50° C.) of a soft segment that is formed by hydrogenating a polymer block formed of isoprene.
However, a retardation film that includes a hydrogenated block copolymer that has the same glass transition temperature Tg1 and glass transition temperature Tg2 as those of the hydrogenated block copolymer disclosed in Patent Literature 5 decreases in retardation when allowed to stand at a temperature of 80° C., and does not exhibit a heat resistance sufficient for an optical film.
Patent Literature 6 to 9 disclose a hydrogenated block copolymer that includes a polymer block derived from an aromatic vinyl compound, and a copolymer block wherein the weight ratio of an aromatic vinyl compound is larger than that of a linear conjugated diene compound.
Patent Literature 6 and 8 disclose a hydrogenated triblock copolymer that has a total styrene content of 80 wt %, wherein the hard segment is a polymer block formed of styrene, and the soft segment is a copolymer block in which the styrene:butadiene weight ratio is 50:50, and the styrene:isoprene weight ratio is 40:20.
However, the hydrogenated triblock copolymer disclosed in Patent Literature 6 has a softening temperature measured by a thermomechanical analyzer (TMA) of 127° C., and the hydrogenated triblock copolymer disclosed in Patent Literature 8 has a softening temperature measured by a thermomechanical analyzer (TMA) of 129° C. (i.e., do not exhibit sufficiently high heat resistance).
Patent Literature 6 and 8 also disclose a hydrogenated pentablock copolymer that has a total styrene content of 80 wt %, wherein the hard segment is a polymer block formed of styrene, and the soft segment is a polymer block formed of isoprene, the hydrogenated pentablock copolymer having a polystyrene-polyisoprene-polystyrene-polyisoprene-polystyrene block configuration. However, the hydrogenated pentablock copolymer has a softening temperature of 116° C., and exhibits a heat resistance lower than that of the hydrogenated triblock copolymer.
Patent Literature 7 discloses a hydrogenated triblock copolymer that has a total styrene content of 85 wt %, wherein the hard segment is a polymer block formed of styrene, and the soft segment is a copolymer block in which the styrene:isoprene weight ratio is 19:15.
However, the hydrogenated triblock copolymer disclosed in Patent Literature 7 has a glass transition temperature Tg measured by DSC analysis of 125.5° C., and does not exhibit sufficiently high heat resistance.
Patent Literature 9 discloses a hydrogenated triblock copolymer that has a total styrene content of 80 wt %, wherein the hard segment is a polymer block formed of styrene, and the soft segment is a copolymer block in which the styrene:isoprene weight ratio is 30:20.
However, the hydrogenated triblock copolymer disclosed in Patent Literature 9 has a softening temperature measured by a TMA of 128° C., and does not exhibit sufficiently high heat resistance.
As described above, a number of pieces of literature disclose a hydrogenated block copolymer wherein the hard segment is a polymer block derived from an aromatic vinyl compound, and the soft segment is a copolymer block derived from a linear conjugated diene compound and an aromatic vinyl compound. However, these hydrogenated block copolymers do not necessarily exhibit a heat resistance sufficient for an optical film used for a liquid crystal display.